Recently, with the increase in the use of various electric and electronic appliances, problems of electromagnetic noises or interferences (EMI) have been on the increase. Such noises may be generally classified into conduction noises and emission or radiation noises. The use of noise filters is a typical measure against conduction noises. As for radiation noises, it is necessary to electromagnetically insulate a prescribed space. To this end, the appliance may be enclosed in a metallic or otherwise electroconductive casing, a metallic plate may be placed between the two circuit boards, or metallic foil may be wrapped around the cable. These measures may provide an adequate electromagnetic shield for the circuit or the power source block, but are unsuitable for shielding electromagnetic radiation which may be produced from the front surface of a display device such as a CRT or a PDP because such measures require an opaque material layer to be placed in front of the display device.
Methods for providing both an electromagnetic shielding effect and a transparency have been previously proposed (see Japanese patent laid open publications Nos. 1-278800 and 5-323101) which are based on forming electroconductive thin film over the surface of a transparent base member by vapor depositing metal or metal oxide.
There have also been proposed electromagnetic shielding materials having highly electroconductive fibers embedded in a transparent base material layer (see Japanese patent laid open publications Nos. 5-327274 and 5-269912), electromagnetic shielding materials having electroconductive resin material containing metallic powder or the like directly deposited or printed on a transparent base board (see Japanese patent laid open publications Nos. 62-57297 and 2-52499), and an electromagnetic shielding material having a transparent resin layer formed over a transparent base board such as polycarbonate board having the thickness of approximately 2 mm, and a copper layer of a mesh pattern formed over the resin layer by electroless plating (see Japanese patent laid open publication No. 5-283889).
According to the methods of forming a thin electroconductive layer by vapor depositing metal or metal oxide onto a transparent base board which were proposed in Japanese patent laid open publications Nos. 1-278800 and 4-323101 and purported to achieve both an electromagnetic shielding capability and a transparency, if the thickness of the electroconductive layer is reduced to a sufficient level (a few hundred .ANG. to 2,000 .ANG.), the surface resistance of the electroconductive layer becomes so large than the shielding effect for the frequency range of 1 MHz to 1 GHz will be less than 20 dB which is significantly lower than the required level of 30 dB or more.
As for the electromagnetic shielding material consisting of a transparent base member having electroconductive fibers embedded therein such as those proposed in Japanese patent laid open publications Nos. 5-327274 and 5-269912, a sufficiently high electromagnetic shielding effect of 40 to 50 dB can be achieved for the frequency range of 1 MHz to 1 GHz, but the fiber diameter which is required for regularly arranging the electroconductive fibers so as to achieve such a shielding effect becomes as large as 35 .mu.m, and the fibers are so visible (which is referred to as "visibility" hereinafter) that the shielding material is quite unfit for application to display devices. In the case of the electromagnetic shielding material made by directly printing electroconductive ink containing metallic powder or the like on a transparent base board, similarly, the line width is no less than 100 .mu.m due to the limitation of printing precision so that the visibility makes the material unsuitable. As for the shielding material formed by forming a transparent resin layer over a transparent base board made of a polycarbonate plate or the like having a thickness in the order of 2 mm, and forming a copper mesh pattern thereon by electroless plating, the surface of the transparent base board is required to be roughened or made coarse so as to achieve a sufficient adhesive power for the electroless plating. This roughening process requires the use of toxic oxidants such as chromic acid and permanganate acid, and can produce a desired result only when the base board is made of ABS resin. According to this method, even when an electromagnetic shielding effect and a transparency are both achieved, the thickness of the transparent base board cannot be reduced to a sufficient level so that the material is not suited to be formed into a sufficiently thin film or web. If the transparent base board has a significant thickness, as it cannot be closely attached to the surface of the display device, there will be a leakage of electromagnetic radiation. Also, in regard to the manufacturing process, because the shielding material cannot be put into the form of rolls, the material tends to be undesirably bulky and the unsuitability of the material to automation causes an increase in the production cost.
The electromagnetic radiation from the front surface of the display device that needs to be shielded is not limited the electromagnetic radiation in the frequency range of 1 MHz to 1 GHz that has to be reduced by 30 dB or more, but the infrared radiation in the wavelength range of 900 to 1,100 nm is also required to be blocked as it interferes with VTR equipment.
In addition to a favorable transparency for visible light, the material is required to have a favorable bonding property which allows it to be closely attached to the display surface so as to effectively shut off electromagnetic radiation, and to achieve a invisibility which makes the presence of the shielding material unnoticeable to the viewer. The bonding property includes the capability of the material to be attached to the surface of any one of a number of widely used polymer plate materials as well as to the surface of glass at a relatively low temperature, and to continue to maintain the close contact with the material over extended periods of time. However, such desirable material that can be formed into web or a roll, and which can sufficiently meet the requirements of the electromagnetic shielding effect, the infrared radiation blocking effect, the transparency, the invisibility, and the bonding property has not been hitherto available.
The part of the surface of the transparent plastic base member from which the electroconductive material has been removed presents an irregular surface because the surface was deliberately formed into an irregular surface in order to increase the adhesion force or because the marks of the reverse surface of the electroconductive material were imprinted on the surface of the transparent plastic base member. Therefore, the irregular reflection on the surface may damage the transparency of the base member, but by evenly applying a resin material layer, having a refraction index close to that of the base member, over such an irregular surface, the irregular reflection is minimized so that the transparency of the base member may be restored. The electroconductive material geometrically patterned on the surface of the transparent plastic base member has such a fine line width that it is practically invisible to naked eyes. The large line spacing also contributes to the invisibility of the electroconductive material.
Also, it is believed that, because the pitch of the geometric pattern is sufficiently smaller than the wavelength of the electromagnetic radiation that is desired to be shielded, a superior shielding performance can be achieved.
Also, the PDP panel, which is one type of flat panel, requires its front surface to be free from any warping. To control such a warping, it is preferable to use a symmetric structure, and is conceivable to attach a pair of transparent plastic sheets onto either side of a bonding agent layer. However, because the front panel of a display device is normally required to have a thickness of 2 to 5 mm in view of the possibility of breaking the glass and the ease of handling, a pressing process is necessary in view of the thermal conduction to the bonding agent in order to produce such an assembly. Additionally, the front panel of the display requires an anti-glare processing or an anti-reflection processing, it is required to be carried out directly on the transparent plastic base sheet, and the production process must be carried out as a batch process. For these two reasons, the production of such an assembly involves the problem that a continuous production process is not possible, and hence the production cost would be undesirably high.